1. Field of the Invention
The present invention relates to a circuit for driving a load in a class AB push-pull manner, such as a loudspeaker drive circuit in an acoustic system, a motor drive circuit in a servo system or an output circuit in an operational amplifier, and a drive method for such a load driving circuit.
2. Description of the Prior Art
There is known such a class AB push-pull drive circuit as shown in FIG. 23, for example. The illustrated circuit comprises an NPN transistor Q.sub.101 and a PNP transistor Q.sub.102, the emitters of which are connected to each other. A positive supply voltage V.sub.cc is applied to the collector of the transistor Q.sub.101 while a negative supply voltage V.sub.ss is applied to the collector of the transistor Q.sub.102. A constant current source Q.sub.103 is connected between the collector and base of the transistor Q.sub.101 while a constant current source Q.sub.104 is connected between the base and collector of the transistor Q.sub.102. Two diodes D.sub.101 and D.sub.102 are further connected in series between the bases of the transistors Q.sub.101 and Q.sub.102. These diodes D.sub.101 and D.sub.102 are in a forward biased direction relative to the P-N junction between the base and emitter of the transistors Q.sub.101 and Q.sub.102. An external voltage V.sub.i is applied to the connecting point of the diodes D.sub.101 and D.sub.102 while the emitters of the transistors Q.sub.101 and Q.sub.102 output a drive current i.sub.0 toward a load in the post-stage (not shown).
The illustrated circuit can make the drive current i.sub.0 class AB, as shown in FIG. 24. More particularly, if it is assumed that the emitter voltage of the transistors Q.sub.101 and Q.sub.102 is V.sub.0, the collector currents i.sub.n and i.sub.p of the transistors Q.sub.0101 and Q.sub.102 will vary relative to V.sub.i -V.sub.0 in such a manner as shown by broken line in FIG. 24. As a result, the drive current i.sub.o =i.sub.n -i.sub.p will vary as shown by solid line in FIG. 24.
FIG. 25 shows another layout of the class AB push-pull drive circuit. This circuit uses an N-channel FET (Field-Effect Transistor) Q.sub.105 in place of the NPN transistor Q.sub.101 as in the prior art shown in FIG. 23 and a P-channel FET Q.sub.106 in place of the PNP transistor Q.sub.102. The sources of the FET Q.sub.105 and Q.sub.106 are connected to each other. The connected sources output a drive current i.sub.0 toward a load in the post-stage (not shown). A positive supply voltage V.sub.DD is applied to the drain of the FET Q.sub.105 while a negative supply voltage V.sub.ss is applied to the drain of the FET Q.sub.106. A constant current source Q.sub.103 is connected between the gate and drain of the FET Q.sub.105 while a constant current source Q.sub.104 is connected between the gate and drain of the FET Q.sub.106. Further, the circuit uses an N-channel FET Q.sub.107 in place of the diode D.sub.101 shown in FIG. 23 and a P-channel FET Q.sub.108 in place of the diode Q.sub.102. The gates and drains of the FETs Q.sub.107 and Q.sub.108 are externally short-circuited to one another, with the sources thereof receiving a voltage V.sub.i. This circuit can also provide such characteristics as shown in FIG. 24.
However, such circuits as shown in FIGS. 23 and 25 cannot be used if the supply voltages V.sub.cc, V.sub.DD or V.sub.ss are low. In other words, the base voltage of the transistor Q.sub.101 or the gate voltage of the FET Q.sub.105 cannot exceed the positive supply voltage V.sub.cc or V.sub.DD. Actually, the base voltage of the transistor Q.sub.101 or the gate voltage of the FET Q.sub.105 will further be limited since the transistor Q.sub.101 or the FET Q.sub.105 has a drop in the voltage between the base and emitter or between the gate and source. Similarly, the base voltage of the transistor Q.sub.102 or the gate voltage of the FET Q.sub.106 cannot be lower than the negative supply voltage V.sub.ss. There is also a drop in the voltage between the base and emitter of the transistor Q.sub.102 or between the gate and source of the FET Q.sub.106. In these prior arts, since the potential difference between the positive and negative supply voltages cannot be fully utilized, the amplitude of the output voltage V.sub.0 is smaller than a level determined by the potential difference. In other words, these circuits of the prior art cannot be driven by a relatively low voltage source.
To overcome such problems, there has been proposed such a class AB push-pull drive circuit as shown in FIG. 26. This circuit comprises a PNP transistor Q.sub.112 and an NPN transistor Q.sub.114, the collectors of which are connected to each other. A positive supply voltage V.sub.cc is applied to the emitter of the transistor Q.sub.112 while a negative supply voltage V.sub.ss is applied to the emitter of the transistor Q.sub.114. The collectors of the transistors Q.sub.112 and Q.sub.114 output a drive current i.sub.0 toward a load in the post-stage, with a voltage being V.sub.0 at this point. If it is assumed that the collector current of the transistor Q.sub.112 is i.sub.p and the collector current of the transistor Q.sub.114 is in, the outputted drive current i.sub.0 becomes i.sub.p -i.sub.n.
The transistors Q.sub.112 and Q.sub.114 are connected to PNP and NPN transistors Q.sub.111, Q.sub.113, respectively. An external voltage V.sub.i is applied to the base of the transistor Q.sub.111 and also to the collector of an NPN transistor Q.sub.118 which forms part of a differential input circuit 101. In addition to the transistor Q.sub.118, the differential input circuit 101 comprises another NPN transistor Q.sub.117 having its emitter connected to that of the first transistor Q.sub.118, a constant current source Q.sub.119 for supplying a constant current to the emitters of the transistors Q.sub.117 and Q.sub.118, and constant current sources Q.sub.120 and Q.sub.121 for supplying constant currents to the collectors of the transistors Q.sub.117 and Q.sub.118. The transistor Q.sub.118 is connected in parallel to two diodes D.sub.111 and D.sub.112 which are connected in series to each other in the forward biased direction relative to the P-N junction between the base and emitter of the transistor Q.sub.118. The diodes D.sub.111 and D.sub.112 receive a constant current from a constant current source Q.sub.122. Thus, the voltage between the base and emitter of the transistor Q.sub.118 is maintained constant. The base of the transistor Q.sub.113 is connected to the collector of the transistor Q.sub.117 which is paired with the transistor Q.sub.118.
The base of the transistor Q.sub.117 is connected to the collector of the PNP transistor Q.sub.115 and also to the emitter of the PNP transistor Q.sub.116. A positive supply voltage V.sub.cc is applied to the emitter of the transistor Q.sub.115 while a negative supply voltage V.sub.ss is applied to the collector of the transistor Q.sub.116. In the circuit of the prior art, therefore, the voltage between the base and emitter of the transistor Q.sub.112 is transferred between the base and emitter of the transistor Q.sub.116, so that the sum of the voltage between the base and emitter of the transistor Q.sub.114 and the voltage between the base and emitter of the transistor Q.sub.112 is applied to the base of the transistor Q.sub.117. As described, the base of the transistor Q.sub.117 receives a voltage relating to two P-N junctions between the diodes D.sub.111 and D.sub.112. The differential input circuit 101 compares the voltages relating to these diodes with the voltages between the base and emitter in the transistors Q.sub.114 and Q.sub.112. Thus, the current i.sub.0 can be driven in class AB.
Unlike the circuit shown in FIG. 23, the circuit of FIG. 26 can be driven by a relatively low supply voltage since the amplitude range of the output voltage V.sub.0 will not be affected by the voltage between the base and emitter in the transistors Q.sub.112 and Q.sub.114 which relate to the output. However, such a circuit of the prior art also raises other problems.
First, the circuit of FIG. 26 produces a phase difference between signals amplified by two push-pull amplification paths, since they provide a large path difference. More particularly, the circuit of FIG. 26 provides two amplification paths, a first path of the base of the transistor Q.sub.111 .fwdarw.the emitter thereof.fwdarw.the base of the transistor Q.sub.112 .fwdarw.the collector thereof and a second path of the base of the transistor Q.sub.111 .fwdarw.the emitter thereof.fwdarw.the base of the transistor Q.sub.115 .fwdarw.the collector thereof.fwdarw.the base of the transistor Q.sub.117 .fwdarw.the collector thereof.fwdarw.the base of the transistor Q.sub.113 .fwdarw.the emitter thereof.fwdarw.the base of a transistor Q.sub.114 .fwdarw.the collector thereof. As can be seen from FIG. 26, the difference between these two signal amplification paths is very large.
Second, the circuit of FIG. 26 includes two closed negative-feedback loops for class AB drive. These closed negative-feedback loops tend to oscillate. More particularly, these two closed negative-feedback loops are a first loop of the base of the transistor Q.sub.111 .fwdarw.the emitter thereof.fwdarw.the base of the transistor Q.sub.115 .fwdarw.the collector thereof.fwdarw.the base of the transistor Q.sub.117 .fwdarw.the emitter thereof.fwdarw.the emitter of the transistor Q.sub.118 .fwdarw.the collector thereof.fwdarw.the base of the transistor Q.sub.111 and a second loop of the base of the transistor Q.sub.113 .fwdarw.the emitter thereof.fwdarw.the base of the transistor Q.sub.116 .fwdarw.the emitter thereof.fwdarw.the base of the transistor Q.sub.117 .fwdarw.the collector thereof.fwdarw.the base of the transistor Q.sub.113. Both of these loops tend to oscillate.